Is wet clay from house backyard good for house plaster??

dejan dejanovic

Posts: 13

posted 9 months ago

Hello members, need your help...
I have finished 3.phase of house building. From the begin I have a plan to use straw and clay for walls. More or less main procedure is clear to me, like mixture, or wall applying, etc.
But, last week I have talked with overview with one guy, like expert, which remind me that I need to use dry and sift clay! In my country 1t of sifted clay comes around 250$, and I think I will needed around 10tons for outside/inside walls.

I'm not sure if any one has try, but has someone try with wet clay, excavated from backyard, directly for plaster mixture?

Hopefully someone with some experience will chime in here, shortly. But, I think it would have a lot to do with how much clay there is in your soil. That dried and sifted clay that they sale for 250 a ton was dug out of the ground somewhere. Does your local area have any traditions of pottery or perhaps brick making? That would be a good indication that you're likely to have suitable clay in your own backyard.

Terry Ruth

Posts: 698

26

posted 9 months ago

Here in the US we have companies like Laguna we get ceramic clay's for beautiful fine grain walls. Not cheap though I can get it from American Clay for less and ready to apply. I'd like to find a place to get different quality clay's at low cost.

It is a science to get the proper mix and to know what families of clay's do what to the mix ask at pottery pro, fired or not.

For the most part an average knowledgeable person should be able to start with clay from the back yard and use it for something for earth scratch and brown coats, the finish is where you can spend some time or money.

I think that Terry is right in using the backyard clay for your scratch and brown coat. I did this and then used bagged pottery clay for the finish coat and it came out very nice. Just experiment with as many recipes as possible and you might find something that you really like and comes for free from your property. Try varying quantities of wheat paste, sand, straw/manure, pigments to see what turns out really nice and won't crack.

I have a friend who did his whole two story straw bale house with common grey clay/silt/soil from his yard (I think he said his soil was 80% clay where he was digging it). I helped do some mudding, and it was very effective mushing into the bales.

Tobias Ber

Posts: 391

Location: Northern Germany (Zone 8a)

13

posted 9 months ago

it depends on your local clay. you might mave to add sand and maybe chopped straw to the mix. it should cling very well to strawbale, when you massage it in.

you need to check if there are stones in your clay. people would let lumps of clay soak over night in water. strain it. add sand and straw. so there s no need to work with dry stuff. but dry stuff will be easier to mix in a cement-mixer like normal plaster.

finishes might be possible. you need to experiment. like making many, many different test batches of 1-2 sqft. little cracks can be filled in with another application of the same stuff. the look of the surface will depend on the size of your sand-grains and technique.

i ll attach a photo of my experiment with dug-up clay, water, sand and short straw (around 1cm). the wall-surface on it is not perfect, it still has residues of acrylic paint on it. the cob mixture might have too much sand. but it seems to work well so far. i smeared it on the wall and flattened it with a small, wet trowel in circular motions. so the surface will look more rough than smoot/polished

DSCN9459.JPG

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Terry Ruth

Posts: 698

26

posted 9 months ago

If you get a hold of some Kaoline it will add luster, shine (gloss or semi), workability to the finish but has little to no binder. Add some pumice, scoria, lava, rock aggregates, straw, hemp, two tones, looks nice to some. Remember just because a test seems to be holding does not mean it will last. Add some heat and moisture cycles depending how harsh the local is. If you get high humidity or wind driven rains on exteriors take a hose with the right nozzle to it. When it comes to adhesion try and get the highest scratch/brown as possible. The two properties to test for is tension and shear. I try and scrape the test mix off the wall with a drywall knife @ 45 degree. If on lathe it's more tension or a pulling or falling off from weight and vapor pressures.

BTW: If the soil is not binding well it can be stabilized with lime (type S, or NHL 3.5 or 5), ashes like fly as pozzalans, portland cement. If you look on American Clays website they have some good trowling techniques, primers, and sealers.

But yes there are so many variables at play here there is no cookie cutter technique for the most part.

80% clay is too high BTW: 10-30% binder should do it or it gets too expansive depending on type. A jar test gives an idea on clay content, a lab soil sieve size, etc, test is better.

I read that portland and lime might hurt the straw, like degrading it through the alkalicity. Is that true?

dejan, did you do a test batch on a strawbale? Just dig out your local clay. Soak it. Knead/tread it until there are no lumps left. With adding water to desired consistency. and then just plaster a straw bale. you might do a few batches, where you mix your clay with sand. Write down the ratio, let it dry (maybe inside to speed things up) and test it.

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dejan dejanovic

Posts: 13

posted 9 months ago

Casie Becker wrote:Hopefully someone with some experience will chime in here, shortly. But, I think it would have a lot to do with how much clay there is in your soil. That dried and sifted clay that they sale for 250 a ton was dug out of the ground somewhere. Does your local area have any traditions of pottery or perhaps brick making? That would be a good indication that you're likely to have suitable clay in your own backyard.

My country, Slovenia (Europe, border with Austria or Italy, Hungary,...) basically lying on clay. Yes, it has also tradition of clay, also brick manufacturing. Actual clay roof bricks are very popular last 10y.
My backyard, earth consist, like that: 50cm is black soil >> 1-1.5 meters is clay >> after is rocky. Check attached picture. Everything below red line is clay...
So, yes I have very good option to dig out clay, and use it. That why I would like to avoid to buy clay, if I have own.
But, to dry clay it would be hard time for me, as I have no proper space for drying. And, it would take ages to dry clay, and sift it.

IMG_0193.JPG

dejan dejanovic

Posts: 13

posted 9 months ago

Daniel Ray wrote:I think that Terry is right in using the backyard clay for your scratch and brown coat. I did this and then used bagged pottery clay for the finish coat and it came out very nice. Just experiment with as many recipes as possible and you might find something that you really like and comes for free from your property. Try varying quantities of wheat paste, sand, straw/manure, pigments to see what turns out really nice and won't crack.

Similar, or even same procedure I will take.
In my country we are often using LIME powder as first layer, to protect straw from moisture, mold, rot,...
Second layer would be rough mixture: ROUGH CLAY + 0-8mm SAND + STRAW + WATER
Third layer would be finish coat: FINE CLAY + 0-4mm SAND + WATER.

So, I guess I could use clay for rough coat directly backyard?

dejan dejanovic

Posts: 13

posted 9 months ago

Roberto pokachinni wrote:I have a friend who did his whole two story straw bale house with common grey clay/silt/soil from his yard (I think he said his soil was 80% clay where he was digging it). I helped do some mudding, and it was very effective mushing into the bales.

So, you did not dry it? When you were dig out clay, you have create mixture right away?
What time/year was when house was done? Asking how plaster is behaving?

Can you please ask your friend about, or if you even might know, about plaster mixture, like what % of materials?

dejan dejanovic

Posts: 13

posted 9 months ago

Tobias Ber wrote:it depends on your local clay. you might mave to add sand and maybe chopped straw to the mix. it should cling very well to strawbale, when you massage it in.

you need to check if there are stones in your clay. people would let lumps of clay soak over night in water. strain it. add sand and straw. so there s no need to work with dry stuff. but dry stuff will be easier to mix in a cement-mixer like normal plaster.

finishes might be possible. you need to experiment. like making many, many different test batches of 1-2 sqft. little cracks can be filled in with another application of the same stuff. the look of the surface will depend on the size of your sand-grains and technique.

i ll attach a photo of my experiment with dug-up clay, water, sand and short straw (around 1cm). the wall-surface on it is not perfect, it still has residues of acrylic paint on it. the cob mixture might have too much sand. but it seems to work well so far. i smeared it on the wall and flattened it with a small, wet trowel in circular motions. so the surface will look more rough than smoot/polished

Great. I like idea for leaving clay in water over night. Like presentation from YouTube...

Yeap, in any case I will work with concrete mixer + plastering machine.
Thanks for PDF. Will read it definitely...

dejan dejanovic

Posts: 13

posted 9 months ago

Terry Ruth wrote:If you get a hold of some Kaoline it will add luster, shine (gloss or semi), workability to the finish but has little to no binder. Add some pumice, scoria, lava, rock aggregates, straw, hemp, two tones, looks nice to some. Remember just because a test seems to be holding does not mean it will last. Add some heat and moisture cycles depending how harsh the local is. If you get high humidity or wind driven rains on exteriors take a hose with the right nozzle to it. When it comes to adhesion try and get the highest scratch/brown as possible. The two properties to test for is tension and shear. I try and scrape the test mix off the wall with a drywall knife @ 45 degree. If on lathe it's more tension or a pulling or falling off from weight and vapor pressures.

BTW: If the soil is not binding well it can be stabilized with lime (type S, or NHL 3.5 or 5), ashes like fly as pozzalans, portland cement. If you look on American Clays website they have some good trowling techniques, primers, and sealers.

But yes there are so many variables at play here there is no cookie cutter technique for the most part.

80% clay is too high BTW: 10-30% binder should do it or it gets too expansive depending on type. A jar test gives an idea on clay content, a lab soil sieve size, etc, test is better.

Nice tips, will play around with your suggestions on straw bale. w
My area, weather over year contains everything: all 4 sessions, summer 50 degree on the sun, winter can be also -20, also period of strong dry wind.

dejan dejanovic

Posts: 13

posted 9 months ago

Tobias Ber wrote:I read that portland and lime might hurt the straw, like degrading it through the alkalicity. Is that true?

dejan, did you do a test batch on a strawbale? Just dig out your local clay. Soak it. Knead/tread it until there are no lumps left. With adding water to desired consistency. and then just plaster a straw bale. you might do a few batches, where you mix your clay with sand. Write down the ratio, let it dry (maybe inside to speed things up) and test it.

I have never hear about lime not recommend for straw. Just opposite, it is defending from organisms.
Will do test within next 2-3 weeks, and I will post results for all of you...

Tobias Ber

Posts: 391

Location: Northern Germany (Zone 8a)

13

posted 8 months ago

this site says: first coat of plaster without sand. just clay and straw

i was always thinking: clay + SAND + fibre ... but we re talking about plaster here, not about structural cob for walls n stuff. any thoughts/experiences?

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Terry Ruth

Posts: 698

26

posted 8 months ago

But yes there are so many variables at play here there is no cookie cutter technique for the most part.

Remember this? I think some people get confused into thinking that here are no variables and it is cookie cutter. If it worked for John & Jane Doe across the world it has to work for me on the other side under entirely different conditions. Further, a pulled pic or website off the internet verifies this theory and everything looks fine other than the fact we have no building life cycle history. There are some basic mix guidelines that are not more than those basic guidelines with variables.

Attaching a scratch coat to straw is entirely different than lime bricks w/acrylic on them. It appears based on the linked guy’s clay content and type he found that a clay and straw mix works for him. Makes sense when attaching to strawbale it his situation. If he had little clay or binder adding lime or Portland cement would give him more scratch binder. If he had too much clay, adding lime would reduce the moisture content (MC) of the mix and make it less expansive, more stable. It also act as a biocide, anti-microbial due to chemistry and lower MC.

Some wood fibers will bond better than others depending on silica (sand) content. Hemp will bond better to lime MGO since it has high sand aggregate content ~80% it can grow in sand, hence “hempcrete (lime/hemp)” has been proven for centuries. Wheat straw ~60% (strawcrete), others 30-60% (limecretes (lime any fiber) all over the world. The fiber/sand aggregate binds to the MGO or clay, as far as the rest “breaking down” due to PH level, I would think that the greater chances of that happening are with high PH or acidic soils. Lime is burnt, inert, and stable, except around hydrochloric acid I think it is IIRC depending on type. As lime adsorbs C02 and moisture it gets harder and harder, clay does not but, clay has a high MC depending on type.

Therefore, the chances of a clay/straw cob mix on bricks with acrylic on them lasting the test of time are low. It is much higher on straw; it can be higher when lime is added depending on clay type. Lime MGO added to clay as binder on lime brick is high sustainability since the MGO binders can react and bind when wetted.

All of these methods should be dried in to lower than 20% MC prior to the next layer or microbials, internal cracks, and voids that can allow moisture in and/or acid rain, effloresces, freeze/thaw failures, life cycle fatigue and failures. One way to reduce that is using lime as in a Type S or NHL 5 on the exterior coat, and control expansion/contraction/ clay content to less than 30% in the scratch/brown or natural silicone/silicate sealers such as siloxane/silane.

What begs the question here is why is the ground so saturated and where is the water coming from. When it comes to soaking or “wetting” clay I believe that means just that the clay only to activate the binder since water is the catalyst or reactor in that case. Too wet a mix can take too long to dry, crack, all the above. What is interesting is as soils dry and the microbes leave heat is generated. Once down to a save MC there is no way to survive for the most part. I think the other myth is people think all they have to do is throw some dirt on the wall and since that is “natural” there can’t be any microbials leaching into my air quality or lungs/resporatory tracks. Not the case natural materials can be just a deadly as any depending on how they are processed.

i was always thinking: clay + SAND + fibre ... but we re talking about plaster here, not about structural cob for walls n stuff. any thoughts/experiences?

Now, I'm also confused. My knowledge tells me that sand is preventing cracks. Someone wrotes that first coat of plaster should actual have cracs. As next fine plaster should filled them, and they would have better adhesion.
I know we need to wait for each coat to dry first. But, I'm thinking what if in nearest future would first coat cracked again. I'm quite sure crack will have same impact on fine plaster, and crack will be visible from outsite...

Tobias Ber

Posts: 391

Location: Northern Germany (Zone 8a)

13

posted 8 months ago

dejan, i was confused also.

i imagine that on strawbale the cracking is low even without sand. ... but you could test that on a straw bale. when you have strawbale, you ll have 1000s of straws sticking out. the coat grabs into it. even if there s some cracking, it would be held together by the strawbale. the cracks would be filled with the next layer. your dug-up clay will contain some sand, did you do the jar test?

maybe they use a second coat with sand and short (1cm) straw and then an even finer finish.

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Terry Ruth

Posts: 698

26

posted 8 months ago

If any mix cracks that means the shrink index of the mix is too high. Filling the cracks will do nothing to stop internal cracks and voids to repeat in the presence of heat and moisture. Internal cracks can cause issues noted in my last post, some not apparent to the naked eye. High levels of straw or fiber will not necessarily stop cracks or slow down the drying rate of the clay. Lime will stabilize such soils, lime self-heals itself and small cracks in the presence of moisture. Rewetting and slow curing is recommended. Clay does not; some clay has bigger internal pores than lime and a bigger moisture holding capacity. The two combined creates a composite mix with benefits of both. Some clays need it, some do not.

i did two additional earthen plaster test batches to that brick wall with some acrylic-paint residue.

1) just dug-up clay

2) that + short straw

both started to crack and peel of the wall within 2 hours. looking like these pictures of puddles drying up in the desert sun. cracking and the sides (of the flakes) oving upwards (away from the wall).

the one with the sand looks good and feels solid to finger-nail scratching. i applied some lime and like to see how that would look like

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Terry Ruth

Posts: 698

26

posted 8 months ago

i applied some lime and like to see how that would look like

That can take years to reach a full cure so don't draw early conclusions. It is one of the few building materials that get better around moisture.

Hi members,
first, sorry for laaaate reply. I have contact Michael from company strawclaywood.com, which have experience, and person has very fast, and nicely answer to my questions about "backyard wet clay".
Here are two replies.

Hello Dejan-

There's no secret; I'm happy to share what I know.

I and many other natural builders in North America do not add any sand to our base coats of earthen plaster, especially when applying it over straw bale, straw-clay, and other low-density walls. (But also more generally than that; I use a sandless straw-clay plaster for daub on wattle walls and in many other applications.) To avoid excessive cracking you have to add A LOT of chopped straw to your plaster; at least as much volume of chopped straw as clay slip. The best length for the chopped straw is a range between 1 and 4 inches. The resulting mix is very sticky, ensuring a good bond with the substrate; it also dries incredibly strong and tough and provides a lot of strength to the wall. I have demolished a straw bale wall in which the bales were held together by nothing other than this straw-clay plaster (no mesh or pins of any sort), about an inch to an inch and a half thick. I was incredibly impressed at how strong the wall and how hard it was to separate the bales from each other.

You can also make earthen plaster by adding sand and less straw; many people do it that way also. You might try both styles and see which you like best. In my opinion the sandless straw-clay plaster is a bit harder to mix but the results are better. There are some tricks to the application, which is a bit different than a sandy plaster. You have to really push a thin coating of the plaster (called a "discovery coat" or smear coat) onto the bales with your hands - trowels don't work. Then you can immediately build the plaster out as thick as you need to protect the bales and straighten your wall. With a stiff straw-clay mix you can easily build out 3 or 4 inches thick in a single application without sagging or much cracking. You want to do this only in dry weather to avoid mold growth. I float the surface with a wooden float and will often leave that as an exterior finish. On interiors I come back later with a finish coat of sand-rich plaster.

good luck!
Michael

Hello Dejan-

The clay does not have to be dry. In fact, I nearly always soak the clay before using it. Soaking makes it stickier and makes the mixing easier. The only reason to sift clay for a base coat is if it has a lot of rocks in it. Rocks that are smaller than the thickness of the plaster you plan to apply are not a problem, unless there are so many of them that they interfere with mixing or application.

If I have access to one, I use a mortar mixer. A cement mixer does not work with a high-fiber mix; the straw just rolls around in a ball and doesn't get thoroughly combined with the clay. A mortar mixer has a paddle that moves independently of the drum, and allows you to mix a very high-straw mixture. With a mortar mixer, add the clay and water to the mixer first and let them mix until you get smooth mud the consistency of pancake batter or a smoothie.

If I don't have access to a mortar mixer, I use an electric drill with a mixing paddle to combine clay and water to the same consistency described above. Then mix in the straw - at least as much chopped straw as clay slip by volume. You can mix this by hand in a wheelbarrow or by foot on a tarp. One relatively easy way to do it is to place alternating layers of clay slip and chopped straw in a wheelbarrow, let it sit for 15 minutes, and then pull material from the bottom of the wheelbarrow. If you get the proportions right, it should almost mix itself.

good luck!

Michael

Bill Bradbury

pollinator

Posts: 684

Location: Richmond, Utah

32

posted 5 months ago

Hello Dejan,

It's great to hear of DIY strawbale homes; I'd love to see more people get informed, get some help and get to it.

I am a professional plasterer and this is how I was taught to plaster by old time masters.

1) We dig our clay dry, put it into cement mixer to roll the big stuff out by tipping it just the right amount and then screen and process wet from there. Line 2x6 forms on the ground with a bed sheet then put window screen over that and pour the 2 water to 1 clay mix (soupy) through the screen. When the water has drained off(after a day or two) pull the sheet up and you will have silky smooth clay stuck to the sheet. Turn into buckets, I use 70 gallon plastic watering troughs.
2) This is only clay not plaster. To make up a good plaster, you must first have a system. The system described by Michael will work, but has serious drawbacks in; attaching cabinets or electrical or whatever, floating over wooden framing members and dealing with structural settling or earthquakes. This why I cover the wood framing with 15# felt and attach metal lath(welded 2x2 mesh). This system allows you to embed dimensional lumber between bales and lath for a very strong attachment point.
3) I use sand in all 3 coats and straw in much smaller portions than is described, but usually like to shoot heavy render with a Tirolessa sprayer which does not like so much straw. The sand should be mostly sharp and just a little of soft sand. The finer the coat, the more soft sand and less sharp.
4) When making mixes, we test them right on the wall and carve proportions into the plaster so we don't get confused. I like lime in the outer coats. Lime and clay often react chemically to make a really great plaster, it depends on the chemical composition of your clay, but most will benefit at least a little bit from the addition of hydrated lime ( no reason for NHL for this).

Rent a mortar mixer for this job, mixing this much plaster by hand or with a drill is just too much work when the work load is already huge. Apply with a wooden float until the final polish, then steel.